Predicting Extremely Low-Cycle Fatigue Crack Initiation Life Of Partially Concrete-Filled Steel Box-Section Bridge Piers

As an improved form of pure steel pier,partially concrete-filled steel bridge piers not only have good ductility,but also avoid premature local buckling at the bottom of steel pier and improve its energy dissipation capacity under severe earthquakes due to partially in-filled concrete.However,under severe earthquakes,another new failure mode of steel piers may appear before the occurrence of local buckling.Under the reciprocating action of earthquake,steel piers undergo three stages of ductile crack initiation,stable crack propagation and rapid crack propagation,and fracture failure occurs in the early cycle,resulting in a sudden drop in bearing capacity of steel piers.This failure mode is called as extremely low-cycle fatigue failure.The object of this study is partially concrete-filled steel box-section bridge piers subjected to a constant vertical load and cyclic lateral loading at the pier top.By using the finite element analysis software package ABAQUS,three-dimensional elasto-plastic finite element models of partially concrete-filled steel box-section bridge piers are established.The factors influencing extremely low-cycle fatigue crack initiation life and failure mode of concrete-filled steel box-section bridge piers are studied.The principal research contents and achievements are as follows:(1)Comparing the numerical results with test results,it is found that the combined hardening model of steel(Lemaitre-Chaboche model)and damaged plasticity model of concrete can accurately predict the hysteretic behavior of partially concrete-filled steel box-section bridge piers.Non-local damage method in Ge model with 2mm×2mm mesh size can be employed to accurately predict the fatigue crack initiation life of partially concrete-filled steel box-section bridge piers.(2)On the premise of fixed normalized slenderness ratio,axial compression ratio and cyclic loading patterns,the effect of flange's normalized width-to-thickness ratio on crack initiation life is investigated.With the increase of flange's normalized width-to-thickness ratio from 0.2 to 0.8,crack initiation life presents a “V” trend,which decreases first and then increases.When crack initiation life reaches the minimum value,flange's normalized width-to-thickness ratio becomes 0.4 in most cases.Q235 and Q345 grade makes no distinct difference in the influence of flange's normalized width-to-thickness ratio on crack initiation life.(3)On the premise of fixed flange's normalized width-tothickness ratio,axial compression ratio and cyclic loading patterns,the effect of pier's normalized slenderness ratio on crack initiation life is investigated.When flange's normalized width-to-thickness ratio is small,crack initiation life decreases with the increase in pier's normalized slenderness ratio.In some cases,when flange's normalized width-tothickness ratio is large,crack initiation life increases with the increase in pier's normalized slenderness ratio.Q235 and Q345 grade makes no difference in the influence of pier's normalized slenderness ratio on crack initiation life.(4)On the premise of fixed flange's normalized width-tothickness ratio,pier's normalized slenderness ratio and cyclic loading patterns,the effect of axial compression ratio on crack initiation life is investigated.As the axial compression ratio increases,the smaller the lateral yield displacement of the specimens is,the smaller the lateral displacement amplitude of the pier top is,resulting in a decrease in the plastic strain amplitude near the pier base.The single D value corresponding to each half-cycle is smaller and crack initiation life becomes larger.Q235 and Q345 grades makes no difference in the influence of axial compression ratio on crack initiation life.(5)Based on Q235 or Q345 grade,empirical formulas for predicting crack initiation life and local buckling life of partially concrete-filled steel box-section bridge piers are proposed.Only when both crack initiation life and local buckling life are taken into consideration,can the failure mode of the piers be judged more accurately.